The present invention relates generally to electrophotography and more particularly to monocomponent developing arrangements that facilitate efficient development of an electrostatic image and provide consistent high quality image output.
Electrophotographic imaging, or xerography is a well-known method of copying or otherwise printing documents. In general, electrophotographic imaging uses a charge-retentive, photosensitive member known as a photoreceptor having a surface that is initially charged uniformly in the dark. The charged surface is then exposed to a light image representation of a desired image to discharge specific areas of the photoreceptor surface, creating a latent electrostatic charge image. The latent electrostatic charge image is developed by applying toner in a developing unit which carries the toner from a toner container to the photoreceptor surface where it adheres to the charge image, providing a visible image. This developed image is then transferred from the photoreceptor surface to a substrate material such as paper, a transparency or the like.
A color electrophotographic image is typically produced by repeating the same process described above for each of several different image exposures using different colored toners and storing each developed image on an accumulator until all desired colors are applied and then transferring the multicolored image to the substrate.
There are several developing systems known in the art that carry the toner to a developing region to develop a latent image. In one system, known as a non-contact or jump developing system, a thin layer of toner particles is applied to a toner support member or developer roller using a leveling member such as a doctor blade. The surface of the developer roller is spaced a small distance from the latent image-bearing surface of the photoreceptor. When toner on the surface of the developer roller is moved into the developing region between the developer roller and the photoreceptor, the surface charges on the latent image areas of the photoreceptor exert electrostatic forces that draw the toner particles toward the latent image areas on the surface of the photoreceptor.
In order to reduce adhesion forces tending to retain the toner particles on the developer roller, particulate additives having a size much smaller than the size of the toner particles are often used. Such particulate additives are retained on the surface of the toner particles and limit the adhesion of the toner particles to the surface of the developer roller.
In such a jump developing system, the spacing between the adjacent surfaces of the developer roller and the photoreceptor may range between about 100 microns and about 500 microns. Due to this very small spacing it is important to assure accurate and uniform control of the thickness of the toner layer on the developer roller. In certain prior art arrangements, a leveling member such as a curved plate or flexible doctor blade extending across the width of the developer roller engages the surface of the developer roller to control the thickness of the toner layer. The Stockman et al. U.S. Pat. No. 6,298,211 discloses a jump monocomponent development arrangement.
One typical prior art doctor blade mounting arrangement is shown in the Takano et al. U.S. Pat. No. 5,303,010 in which a rigid doctor blade is mounted by clamping screws to a support plate having adjustment slots to control the spacing between the blade and the developer roller and the support plate is in turn mounted by screws to mounting members. The Kobayashi et al. U.S. Pat. No. 5,006,898 discloses a rigid doctor blade affixed to a support member by screws along with a flexible plate for spreading the toner on the support member and the Sakaguchi U.S. Pat. No. 5,602,631 discloses a toner leveling member in the form of a curved plate which confirms the toner layer to a desired thickness. The Sato U.S. Pat. No. 5,752,146 shows a flexible doctor blade for regulating the thickness of a layer of toner particles having a size in a range from 6xcexc to 12xcexc which are coated with particles of an additive such as silica having a size in a range from 10 nm to 30 nm to limit adhesion and improve toner flow and other toner characteristics.
The United States patents to Chatterji et al., U.S. Pat. No. 3,720,617, Hikaki No. 5,066,588, and Bertfeldt No. 5,691,097 also disclose addition of silica particles such as fused silica particles to improve toner properties.
Typically, a fixed leveling member or a flexible doctor blade that is used as a leveling member in a jump development system applies pressure to the toner in order to spread a uniform layer on the surface of the developer roller. Unless the pressure applied to the toner particles by the leveling member is carefully controlled, however, small additive particles such as silica or other additive materials can be forced into the surface of the toner particles, altering their characteristics and reducing the effectiveness of the toner.
Accordingly, it is an object of the present invention to provide a monocomponent developing arrangement for electrophotography which overcomes disadvantages of the prior art.
Another object of the invention is to provide a monocomponent developing arrangement for electrophotography which has improved characteristics and minimizes costs.
These and other objects of the invention are achieved by providing a monocomponent developing arrangement in which the toner has particulate additives of two different size ranges and the ratio of concentrations of the different size range additives optimizes surface coverage of the additives on the toner particles to achieve improved characteristics.
In a preferred embodiment of the invention a developing arrangement for electrophotography utilizes a monocomponent developer with particulate additives of two different size ranges along with a developer roller which carries the toner to a development region where toner particles are selectively attracted to an electrostatic charge image on a photoreceptor surface spaced from the developer roller so as to cause jump development of the charge image and a leveling member such as a doctor blade spreads the toner particles on the surface of the developer roller prior to entering the development region to provide substantially uniform coverage of the surface of the development roller with toner particles bearing additive particles of two different size ranges.
According to one preferred embodiment the toner particles have a mean diameter in a range from about 8 microns to about 14 microns and preferably about 12 microns, and are coated with large additive particles having a mean diameter in a range from about 20 nm to about 50 nm, and preferably about 40 nm, and small additive particles having a mean diameter in a range from about 6 nm to about 12 nm, and preferably about 10 nm, with the large additive particles being supplied in an amount sufficient to produce surface coverage of the toner particles in a range from about 5 percent to about 50 percent and the small additive particles being present in an amount sufficient to produce surface coverage of the toner particles in a range from about 50 percent to about 150 percent.
With this arrangement, it has been found that the toner particle characteristics are not changed significantly by pressure applied by a leveling member such as a doctor blade used to control the height of the layer of toner particles applied to the developer roller and avoid degradation of coatings applied to the toner particles such as coatings of charge control materials are avoided even with significant variations in pressure applied by the leveling member. In this regard, it is believed that the smaller area ratio of large additive particles to small additive particles on the surface of the toner particles is important to inhibit or preclude embedding of the small additive particles into the body of the toner particles even with significant variations in pressure applied by the leveling member.